The magic of 3D printing has touched most of us in some way by now, as schools in many countries are establishing programs, libraries offer makerspaces for patrons, and designers like architects show us their often bold new 3D printed works made possible by accessible new technology. Some researchers, designers, and engineers not only envision the next step, however, but have already made strides in 4D innovation—characterized by parts that are not only highly functional but may be smart enough to morph into different shapes or textured on an as-needed basis.

Researchers from both Southwest Jiaotong University and Georgia Institute of Technology discuss their findings on exploring both 3D and 4D printing via UV-assisted direct-ink write printing. While noting that traditional 3D printing processes make use of typical materials like ABS, PLA, and more, the researchers looked toward epoxy for more expansive uses which may require coating and adhesives. Epoxy is also known for being mechanically strong, and more resistant in the face of temperature and chemicals—qualities that make it suitable for applications like aerospace.

Previously epoxy has presented challenges with strain break and affordability, but 3D printing with direct ink writing capability has been more successful with the use of nanoparticles that add a ‘shear-thinning effect.’ The thermal curing process can be an issue though if not performed at lower temperatures, and with close control and monitoring.

High temperatures may lead to warping of the printed object, though, and this alternative preparation of ingredients for the direct-writing ink has been considered ‘tedious,’ according to previous researchers. Others have developed DIW processes with UV curing. It was successful in applications for creating items such as conductive spring coils and freestanding nanocomposite strain sensors, but there were still significant challenges such as clogging, brittleness, and printed parameter issues.

The research team created a new method, still relying on UV-assistance for curing, but in two stages:

“A new resin containing rapid photocurable resin and thermally curable epoxy oligomer is reinforced with fumed SiO2, which can be utilized as ink for DIW printing,” state the researchers. “Each layer is printed followed by ex situ UV curing, which can eﬃciently avoid nozzle clogging. The flexible network formed by the UV curable resin can hold the shape of the part very well even at an elevated temperature.”

“After DIW printing, the part with the complex structure is moved into a heating oven and thermally cured similar to conventional epoxy resin. Moreover, good interfacial bonding can be achieved by forming chemical bonds between diﬀerent filaments leading to isotropic mechanical properties. This two-stage curing process enables the fabrication of interpenetrating polymer network (IPN) epoxy composites, which show high toughness with tunable mechanical properties. The printed epoxy composite also shows a good shape memory eﬀect with a high shape fixity ratio, shape recovery ratio, and cycling stability.”

Schematic illustration of the 3D printing epoxy composite material

With the new DIW method, they printed:

The researchers state that lower speed is an issue with this technique, but other benefits make up for that, such as ‘excellent interfacial bonding’ of materials and ‘widely tunable mechanical properties’ that are apparent in the post-curing stage. In 3D printing with epoxy composites, one layer of material was deposited with DIW, and then it was UV cured for ten seconds. This is repeated for each layer, allowing for printing of parts with complex geometries, later cured for two hours and post-cured for one hour.

“After that, the epoxy oligomer in the first network was polymerized to form an IPN with highly enhanced mechanical properties,” state the researchers.

Photographs and SEM images of the 3D printing with photo and thermal cure results of epoxy composites. The photos above the dashed line show the printed structures with the photo cure, and the photos under the dashed lines show the structures with two-stage cures (photo cure and subsequent thermal cure). (a) Square-shaped lattice structure; (b) gear wheel; (c) spiral swirl bowl; (d) 3-links trophy; (scale bars in a–d are 6 mm); (e) lattice structure with a single-layered wall and its enlarged SEM images.

The nanocomposite ink, measured with a viscometer, began to exhibit shear-thinning behavior as silica was added, allowing for successful extrusion. In testing, the team created numerous complex structures with a 22 GA nozzle (0.41 mm inner diameter). Not only was 3D printing with their ink sufficient, but they deemed the results to be excellent. Along with this, they began 3D printing with a focus on shape memory, testing their efforts on a 3D printed logo that responded within ten seconds after being immersed in a hot oil bath. Results were the same with a printed test strip also.

With a wide choice of UV-curable resin, thermal curing resin and nanoparticles, this UV-assisted DIW 3D printing via a two-stage curing method can broaden the implementation of 3D printing to directly fabricate thermosetting materials with tunable and enhanced properties for high performance and functional applications,” concluded the researchers. If commercialized this kind of a process would have a broad range of applications in light-based applications.

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